1. Field of the Invention
The present invention relates to a Ni base alloy solid wire for welding for the purpose of welding a Ni base alloy which has a Ni-30Cr based composition.
2. Description of the Related Art
For pressure vessels such as reactor vessels and steam generators constituting pressurized light-water reactors for nuclear power generation, a Ni-30Cr based alloy as a constituent material for the vessels is adopted as a countermeasure against SCC (stress corrosion cracking) which is problematic in the case of conventional Ni-15Cr based alloys. Welding of high-pressure vessels requires corrosion resistance comparable to that of the base material, and a filler metal is thus required which has the same constituent as the base material.
However, when a Ni-30Cr based filler metal is used for overlay welding or joint welding, the inside of weld with deposit metal piled up by multi-pass welding has a problem that cracks are likely to be caused. This intergranular cracking is distinguished from solidification cracking caused in the process of solidification of weld metal, referred to as “ductility dip cracking”, and have a property of being caused in a temperature range in which solidification is completed. This ductility dip cracking refers to a phenomenon in a weld metal of high Cr containing Ni base alloy containing Cr of about 30% or more, in which repeating reheat during welding causes coarse Cr carbide to be precipitated at crystal grain boundaries to lower the grain boundary strength, that is, the bonding strength between adjacent crystal grains, as a result of which the grain boundaries open when a tensile thermal stress or a shear thermal stress loads on the grain boundaries.
As a conventional art for preventing the ductility dip cracking, a rare-earth metal is added in the case of Patent Document 1. Patent Document 1 discloses a Ni base alloy for welding, which contains C: 0.15 mass % or less, Ni: 30.0 to 80.0 mass %, Si: 1.00 mass % or less, Mn: 1.5 mass % or less, Cr: 14.0 to 31.0 mass %, Fe: 51 mass % or less, R (however, R represents at least one of rare-earth materials): 0.05 mass % or less, P: 0.030 mass % or less, and S: 0.015 mass % or less, or a Ni base alloy for welding, which contains C: 0.05 mass % or less, Ni: 58 mass %, Si: 0.5 mass % or less, Mn: 0.5 mass % or less, Cr: 27.0 to 31.0 mass %, Fe: 7.0 to 11.0 mass %, R (however, R represents at least one of rare-earth materials): 0.01 to 0.02 mass %, P: 0.030 mass % or less, S: 0.015 mass % or less, and Cu: 0.5 mass % or less.
In addition, as a conventional art for preventing the ductility dip cracking, Patent Documents 2, 3, and 4 define the ranges for constituents such as P, S, Al, Ti, Nb, and Zr. More specifically, Patent Document 2 discloses, as a nickel-chromium-iron alloy for use in the production of weld deposit, an alloy containing Cr: about 27 to 31.5 mass %, Fe: about 7 to 11 mass %, C: about 0.005 to 0.05 mass %, Mn: about 1.0 mass % or less, Nb: about 0.60 to 0.95 mass %, Si: less than 0.50 mass %, Ti: 0.01 to 0.35 mass %, Al: 0.01 to 0.25 mass %, Cu: less than 0.20 mass %, W: less than 1.0 mass %, Mo: less than 1.0 mass %, Co: less than 0.12 mass %, Ta: less than 0.10 mass %, Zr: about 0.10 mass % or less, S: less than 0.01 mass %, B: less than 0.01 mass %, and P: less than 0.02 mass %, and with the balance including Ni and impurities.
In addition, Patent Document 3 discloses a filler metal for a Ni base high Cr alloy, which has a composition containing C: 0.04 mass % or less, Si: 0.1 to 0.5 mass %, Mn: 0.2 to 1 mass %, Cr: 28 to 31.5 mass %, Mo: 0.5 mass % or less, Cu: 0.3 mass % or less, Nb: 0.1 mass % or less, Al: 0.5 to 1.1 mass %, Ti: 0.5 to 1 mass %, Al+Ti: 1.5 mass % or less, Fe: 7 to 11 mass %, and up to two of W and V at 0.05 to 0.5 mass % in total, and further including Co: 0.1 mass % or less, P: 0.02 mass % or less, S: 0.015 mass % or less, O: 0.1 mass % or less, and N: 0.03 to 0.3 mass % as inevitable impurities, with the balance composed of Ni.
Furthermore, Patent Document 4 discloses a filler metal of a Ni base high Cr alloy, which has a composition containing C: 0.04 weight % or less, Si: 0.01 to 0.5 weight %, Mn: 7 weight % or less, Cr: 28 to 31.5 weight %, Nb: 0.5 weight % or less, Ta: 0.005 to 3.0 weight %, Fe: 7 to 11 weight %, Al: 0.01 to 0.4 weight %, Ti: 0.01 to 0.45 weight %, and V: 0.5 weight % or less, and containing P: 0.02 weight % or less, S: 0.015 weight % or less, O: 0.01 weight % or less, and N: 0.002 to 0.1 weight % as inevitable impurities, with the balance composed of Ni, and further discloses that the composition may contain one or more selected from B, Zr, and rare-earth elements: 0.01 weight % or less, and may further contain Ca: 0.01 weight % or less and Mg: 0.01 weight % or less.
On the other hand, welded joints in the case of carrying out butt welding and fillet welding between Ni base alloys or between combined different materials of a Ni base alloy and a carbon steel have a problem of constraint required for equipment designs, because the tensile strength of the weld metal is inferior as compared with the base material. Thus, for the purpose of improving the tensile strength, a welding material of a high Cr containing Ni base alloy is disclosed which has Al and Ti each added at 0.5 to 3.0 mass % (Patent Document 5). Patent Document 5 discloses a welding material of a high Cr containing Ni base alloy, which has a composition containing C: 0.04 mass % or less, Si: 0.50 mass % or less, Mn: 1.00 mass % or less, Cr: 28.0 to 31.5 mass %, Mo: 0.50 mass % or less, Fe: 7.0 to 11.0 mass %, Cu: 0.30 mass % or less, Nb+Ta: 0.10 mass % or less, Al: 0.5 to 3.0 mass %, and Ti: 0.5 to 3.0 mass %, and further including P: 0.020 mass % or less and S: 0.015 mass % or less as inevitable impurities, with the balance composed of Ni.
[Prior Art Documents]
[Patent Documents]
    [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-288500    [Patent Document 2] Japanese Patent Application Laid-Open No. 2003-501557    [Patent Document 3] Japanese Patent Application Laid-Open No. 8-174269    [Patent Document 4] WO2005/070612    [Patent Document 5] Japanese Patent Application Laid-Open No. 2010-172952